Universal Strategy To Reduce Noise Current for Sensitive Organic Photodetectors

Atto-Scale Noise Near-Infrared Organic Photodetectors Enabled by Controlling Interfacial Energetic Offset through Enhanced Anchoring Ability

The near-infrared (NIR) sensor technology is crucial for various applications such as autonomous driving and biometric tracking. Silicon photodetectors (SiPDs) are widely used in NIR applications; however, their scalability is limited by their crystalline properties. Organic photodetectors (OPDs) have attracted attention for NIR applications owing to their scalability, low-temperature processing, and notably low dark current density (JD), which is similar to that of SiPDs. However, the still high JD (at NIR band) and few measurements of noise equivalent powers (NEPs) pose challenges for accurate performance comparisons. This study addresses these issues by quantitatively characterizing the performance matrix and JD generation mechanism using electron-blocking layers (EBLs) in OPDs. The energy offset at an EBL/photosensitive layer interface determines the thermal activation energy and directly affects JD. A newly synthesized EBL (3PAFBr) substantially enhances the interfacial energy barrier by forming a homogeneous contact owing to the improved anchoring ability of 3PAFBr. As a result, the OPD with 3PAFBr yields a noise current of 852 aA (JD = 12.3 fA cm⁻2 at V → -0.1 V) and several femtowatt-scale NEPs. As far as it is known, this is an ultralow of JD in NIR OPDs. This emphasizes the necessity for quantitative performance characterization.

High-Speed and Ultrasensitive Solar-Blind Ultraviolet Photodetectors Based on In Situ Grown β-Ga2O3 Single-Crystal Films

Deep-level defects in β-Ga2O3 that worsen the response speed and dark current (Id) of photodetectors (PDs) have been a long-standing issue for its application. Herein, an in situ grown single-crystal Ga2O3 nanoparticle seed layer (NPSL) was used to shorten the response time and reduce the Id of metal-semiconductor-metal (MSM) PDs. With the NPSL, the Id was reduced by 4 magnitudes from 0.389 μA to 81.03 pA, and the decay time (τd1/τd2) decreased from 258/1690 to 62/142 μs at -5 V. In addition, the PDs with the NPSL also exhibit a high responsivity (43.5 A W-1), high specific detectivity (2.81 × 1014 Jones), and large linear dynamic range (61 dB) under 254 nm illumination. The mechanism behind the performance improvement can be attributed to the suppression of the deep-level defects (i.e., self-trapped holes) and increase of the Schottky barrier. The barrier height extracted is increased by 0.18 eV compared with the case without the NPSL. Our work contributes to understanding the relationship between defects and the performance of PDs based on heteroepitaxial β-Ga2O3 thin films and provides an important reference for the development of high-speed and ultrasensitive deep ultraviolet PDs.

Green-Solvent-Processed High-Performance Broadband Organic Photodetectors

Solution-processed organic photodetectors with broadband activity have been demonstrated with an environmentally benign solvent, ortho-xylene (o-xylene), as the processing solvent. The organic photodetectors employ a wide band gap polymer donor PBDB-T and a narrow band gap small-molecule non-fullerene acceptor CO1-4F, both dissolvable in o-xylene at a controlled temperature. The o-xylene-processed devices have shown external quantum efficiency of up to 70%, surpassing the counterpart processed with chlorobenzene. With a well-suppressed dark current, the device can also present a high specific detectivity of over 1012 Jones at -2 V within practical operation frequencies and is applicable for photoplethysmography with its fast response. These results further highlight the potential of green-solvent-processed organic photodetectors as a high-performing alternative to their counterparts processed in toxic chlorinated solvents without compromising the excellent photosensing performance.

Flexible all-organic photodetectors via universal water-assisted transfer printing

Transfer printing of small-molecular organic semiconductors often faces challenges due to surface adhesion mismatch. Here, we developed a sacrificing-layer-assisted transfer printing technique for the deposition of small-molecular thin films. High-boiling-point ethylene glycol (EG) was doped in aqueous solution poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) as the sacrificing layer to manipulate residual water in film, which allowed chlorobenzene solution to spontaneously spread and form uniform film. The residual water guaranteed film delamination from the stamp, allowing for its transfer onto various substrates and seeding layers. As a proof of concept, laterally conductive organic photodetectors using recyclable EG-PEDOT:PSS electrodes and a small-molecular active layer were consecutively fabricated via transfer printing in ambient air. The resulting device exhibited a high on/off ratio of 711 and a fast rise time of 0.5 ms. Notably, the polymer electrode and the bulk heterojunction demonstrated unique repairability and recyclability.

Efficient Near-Infrared Organic Photodetectors with Spectral Response up to 1600 nm for Accurate Alcohol Concentration Detection

The development of near-infrared organic photodetectors (NIR-OPDs) in 1000-1700 nm is essential for medical monitoring, food quality inspection, machine vision, and biomedical imaging. However, when solving the high dark current density (J_D) in bulk-heterojunction (BHJ) NIR-OPDs based on narrow-bandgap systems, it is often accompanied by photocurrent loss, which is a great challenge in achieving high-performance NIR-OPDs. Here, an ideal hybrid pseudo-PHJ (planar-heterojunction)/BHJ structure is proposed to overcome this challenge, which is introducing the N2200 layer between the cathode and BHJ. The introduction of the N2200 raises the external charge injection barrier and reduces the trap density, thus achieving significant suppression of J_D (6.22 × 10^-7 A cm^-2 at -0.2 V bias, about 2 orders of magnitude lower compared to the BHJ NIR-OPDs). Meanwhile, the hybrid structure combines the advantages of PHJ and BHJ, thus maintaining a high photocurrent, resulting in responsivity and detectivity of 18.71 mA W^-1 and 4.19 × 10¹⁰ Jones, respectively, at 1400 nm at -0.2 V bias, which is superior to the performance of BHJ NIR-OPDs. And the hybrid structured NIR-OPDs are proven to rapidly quantify the alcohol content of mixtures to within 2% accuracy, which exhibits great potential for application in the food and pharmaceutical industries.

High responsivity of VIS-NIR photodetector based on Ag2S/P3HT heterojunction

Ag₂S quantum dot (QD) photodetectors (PDs) have attracted a lot of attention in the field of imaging system and optical communication. However, the current Ag₂S PDs mainly works in the near-infrared band, and its detection ability in the visible band remains to be strengthened. In this paper, we used poly(3-hexylthiophene) (P3HT) with high carrier mobility and Ag₂S QDs to construct heterojunction PD. Stronger absorption in blends with polymer P3HT compared to single Ag₂S QDs. The optical absorption spectra show that the Ag₂S/P3HT has strong light absorption peak at 394 and 598 nm. The results show that P3HT significantly enhances the absorption of Ag₂S QDs from the visible to near-infrared band. The output characteristics, transfer characteristics and fast switching capability of the device at 405 nm, 532 nm and 808 nm were tested. The device has the responsivity of 6.05 A W^-1, 83.72 A W^-1and 37.31 A W^-1under 405 nm, 532 nm and 808 nm laser irradiation. This work plays an important role in improving the detection performance of Ag₂S QDs and broadening its applications in photoelectric devices for weak light and wide spectrum detection.

Identification of the Origin of Ultralow Dark Currents in Organic Photodiodes

Organic bulk heterojunction photodiodes (OPDs) attract attention for sensing and imaging. Their detectivity is typically limited by a substantial reverse bias dark current density (J_d ). Recently, using thermal admittance or spectral photocurrent measurements, J_d has been attributed to thermal charge generation mediated by mid-gap states. Here, the temperature dependence of J_d in state-of-the-art OPDs is reported with J_d down to 10^-9  mA cm^-2 at -0.5 V bias. For a variety of donor-acceptor bulk-heterojunction blends it is found that the thermal activation energy of J_d is lower than the effective bandgap of the blends, by ca. 0.3 to 0.5 eV, but higher than expected for mid-gap states. Ultra-sensitive sub-bandgap photocurrent spectroscopy reveals that the minimum photon energy for optical charge generation in OPDs correlates with the dark current thermal activation energy. The dark current in OPDs is attributed to thermal charge generation at the donor-acceptor interface mediated by intra-gap states near the band edges.

High-detectivity organic photodetectors with double bulk heterojunction enabled by water transfer printing

Suppressing the dark current is an effective strategy to boost the detection capability of organic photodetectors (OPDs). In this Letter, the water transfer printing method is demonstrated in double bulk heterojunction (BHJ) OPDs, which is solvent-independent rather than the traditional sequential spin-coating method, enabling the elimination of the negative effects of solvents on the underlying film and the suppressing of the dark current. As a result, a photo detectivity up to 10¹² Jones was obtained in the wide spectral range of 400-900 nm with a small working area of 3 mm².

A New Dioxasilepine-Aryldiamine Hybrid Electron-Blocking Material for Wide Linear Dynamic Range and Fast Response Organic Photodetector

A new dioxasilepine and aryldiamine hybrid material DPSi-DBDTA is designed to act as the electron-blocking layer (EBL) for vacuum-processed organic photodetector (OPD). The O-Si-O-linked cyclic structure leads DPSi-DBDTA to have dipolar character, high LUMO, and good thermal and morphology stability suitable for vacuum deposition. An initial trial with C₆₀-based single active layer OPD device manifests the superior capability of DPSi-DBDTA for dark current suppression compared to the typical aryldiamines. Here, the bare and MoO₃-doped DPSi-DBDTA is further examined as EBLs for the visible light responsive OPD comprising DTDCPB/C₇₀ bulk heterojunction (BHJ) as the active layer. In sync with the result of C₆₀-based OPD, the low dark current density and high specific detectivity D* (7.085 × 10¹² cm Hz^1/2 W^-1) are achieved. The device with 5% MoO₃-doped EBL can exhibit a wide linear dynamic range (LDR) up to 154.166 dB, which is attributed to suppression of both dark current density and carrier recombination. Additionally, the devices also manifest fast time-resolved performance in both frequency and transient response measurements. Especially for the device with 20% MoO₃-doped EBL, a wide cutoff frequency response 692.047 kHz and record-high transient response demonstrating ≤0.683 μs for transient photovoltage (TPV) and ≤0.478 μs for transient photocurrent (TPC) have been realized, which is possibly owing to the balance of mobility that mitigates the damage from traps. Such submicrosecond response is comparable with the state-of-the-art perovskite-PDs and Si-PDs.

Ultralow dark current in near-infrared perovskite photodiodes by reducing charge injection and interfacial charge generation

Metal halide perovskite photodiodes (PPDs) offer high responsivity and broad spectral sensitivity, making them attractive for low-cost visible and near-infrared sensing. A significant challenge in achieving high detectivity in PPDs is lowering the dark current density (J_D) and noise current (i_n). This is commonly accomplished using charge-blocking layers to reduce charge injection. By analyzing the temperature dependence of J_D for lead-tin based PPDs with different bandgaps and electron-blocking layers (EBL), we demonstrate that while EBLs eliminate electron injection, they facilitate undesired thermal charge generation at the EBL-perovskite interface. The interfacial energy offset between the EBL and the perovskite determines the magnitude and activation energy of J_D. By increasing this offset we realized a PPD with ultralow J_D and i_n of 5 × 10^-8 mA cm^-2 and 2 × 10^-14 A Hz^-1/2, respectively, and wavelength sensitivity up to 1050 nm, establishing a new design principle to maximize detectivity in perovskite photodiodes.

Fast Response Organic Tandem Photodetector for Visible and Near-Infrared Digital Optical Communications

Organic photodetectors (OPDs), which usually work as photodiodes, photoconductors, or phototransistors, have emerged as candidates for next-generation light sensing. However, low response speed caused by low carrier mobility and resistance-capacitance (RC) time constant, severely hinders the commercialization of OPDs. Herein, the authors demonstrate a state-of-the-art OPD with a record response speed of 146.8 ns by employing tandem structure to simultaneously reduce both the carrier transit time and RC time constant of the device, which is faster than that of previously reported OPDs as far as they know. Moreover, benefitting from the multi-level barrier enhancement and voltage division engendered by tandem structure, an ultralow noise current of 7.82 × 10^-14 A Hz^-1/2 is obtained, as well as a wide detection range in 300-1000 nm. In addition, the tandem OPDs are successfully integrated into the optical communication system as signal receivers, demonstrating the precise digital signal communication from visible to near-infrared light. It is believed that tandem OPDs have promising application potential in the wireless transmission system.

Electrically Switchable Color-Selective Organic Photodetectors for Full-Color Imaging

The present full-color imaging techniques rely on the use of broadband inorganic photodetectors with dedicated color filters, which is one of the practical challenges for large-area, flexible, and high-solution imaging applications. The development of high-performance color-selective photodetectors is one of the key solutions to overcome this challenge. In this work, an electrically switchable color-selective organic photodetector (OPD) comprising a double organic bulk heterojunction structure has been developed for full-color imaging. The color-selective sensing capability over the visible spectrum ranges can be realized by controlling the bias across the OPD, achieving a high responsivity of ∼200 mA/W, a large linear dynamic range of 122 dB, a viewing angle of 120°, and a -3 dB cutoff frequency of >50 kHz. A full-color imaging function has been demonstrated using electrically switchable red-, green-, and blue-color selective OPD sensors with an excellent operational stability. The results of this work provide a practical solution for applications in high-resolution full-color imaging and artificial vision.

Carrier Blocking Layer Materials and Application in Organic Photodetectors

As a promising candidate for next-generation photodetectors, organic photodetectors (OPDs) have gained increasing interest as they offer cost-effective fabrication methods using solution processes and a tunable spectral response range, making them particularly attractive for large area image sensors on lightweight flexible substrates. Carrier blocking layers engineering is very important to the high performance of OPDs that can select a certain charge carriers (holes or electrons) to be collected and suppress another carrier. Carrier blocking layers of OPDs play a critical role in reducing dark current, boosting their efficiency and long-time stability. This Review summarizes various materials for carrier blocking layers and some of the latest progress in OPDs. This provides the reader with guidelines to improve the OPD performance via carrier blocking layers engineering.

Smart Strategy: Transparent Hole-Transporting Polymer as a Regulator to Optimize Photomultiplication-type Polymer Photodetectors

Photomultiplication-type polymer photodetectors (PM-PPDs) were fabricated with hole-only transport active layers containing polymer(s): [6,6]-phenylC61-butyric acid methyl ester (PC₆₁BM) with a weight ratio of 100:2. The rather less PC₆₁BM content in active layers prefers to generate a large amount of isolated electron traps surrounded by polymers. Photogenerated electrons prefer to be trapped by the isolated PC₆₁BM due to the lack of continuous electron-transport channels. The trapped electrons by the isolated PC₆₁BM close to the Al electrode would like to seduce hole tunneling injection. The transparent polymer poly[N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)benzidine] (poly-TPD) was incorporated as a regulator to improve hole mobility (μ_h) and adjust the trapped-electron distribution in active layers, leading to the enhanced performance of PM-PPDs. The optimal PM-PPDs were achieved using poly(3-hexylthiophene) (P3HT):poly-TPD:PC₆₁BM (80:20:2, wt/wt/wt) as active layers. External quantum efficiency (EQE) values at 620 nm are 3900 and 1250% for PM-PPDs based on P3HT:poly-TPD:PC₆₁BM (80:20:2, wt/wt/wt) and P3HT:PC₆₁BM (100:2, wt/wt) under -10 V applied voltage, respectively. The EQE at 620 nm of optimal PM-PPDs is improved from 650 to 63,000% along with the applied voltage increase from -5 to -20 V. This work provides a new strategy of using transparent polymer with large μ_h as a regulator for EQE and response speed improvement, as well as the flattened EQE spectral shape of PM-PPDs.

Understanding and Countering Illumination-Sensitive Dark Current: Toward Organic Photodetectors with Reliable High Detectivity

Continuously enhanced photoresponsivity and suppressed dark/noise current combinatorially lead to the recent development of high-detectivity organic photodetectors with broadband sensing competence. Despite the achievements, reliable photosensing enabled by organic photodetectors (OPDs) still faces challenges. Herein, we call for heed over a universal phenomenon of detrimental sensitivity of dark current to illumination history in high-performance inverted OPDs. The phenomenon, unfavorable to the attainment of high sensitivity and consistent figures-of-merit, is shown to arise from exposure of the commonly used electron transport layer in OPDs to high-energy photons and its consequent loss of charge selectivity via systematic studies. To solve this universal problem, "double" layer tin oxide as an alternative electron transport layer is demonstrated, which not only eliminates the inconsistency between the initial and after-illumination dark current characteristics but also preserves the low magnitude of dark current, good external quantum efficiency, and rapid transient response.

Reverse dark current in organic photodetectors and the major role of traps as source of noise

Organic photodetectors have promising applications in low-cost imaging, health monitoring and near-infrared sensing. Recent research on organic photodetectors based on donor-acceptor systems has resulted in narrow-band, flexible and biocompatible devices, of which the best reach external photovoltaic quantum efficiencies approaching 100%. However, the high noise spectral density of these devices limits their specific detectivity to around 1013 Jones in the visible and several orders of magnitude lower in the near-infrared, severely reducing performance. Here, we show that the shot noise, proportional to the dark current, dominates the noise spectral density, demanding a comprehensive understanding of the dark current. We demonstrate that, in addition to the intrinsic saturation current generated via charge-transfer states, dark current contains a major contribution from trap-assisted generated charges and decreases systematically with decreasing concentration of traps. By modeling the dark current of several donor-acceptor systems, we reveal the interplay between traps and charge-transfer states as source of dark current and show that traps dominate the generation processes, thus being the main limiting factor of organic photodetectors detectivity.

Broadband Ultraviolet Self-Powered Photodetector Constructed on Exfoliated β-Ga2O3/CuI Core-Shell Microwire Heterojunction with Superior Reliability

A heterojunction is an essential strategy for multispectral energy-conservation photodetection for its ability to separate photogenerated electron-hole pairs and tune the absorption edge by selecting semiconductors with appropriate bandgaps. A broadband ultraviolet (200-410 nm) self-powered photodetector is constructed on the exfoliated β-Ga₂O₃/CuI core-shell microwire heterostructure. Benefiting from the photovoltaic and photoconductive effects, our device performs an excellent ultraviolet (UV) discriminability with a UVC/visible rejection ratio (R₂₂₅/R₆₀₀) of 8.8 × 10³ and a UVA/visible rejection ratio (R₄₀₀/R₆₀₀) of 2.7 × 10², and a self-powered photodetection with a responsivity of 8.46 mA/W, a detectivity of 7.75 × 10¹¹ Jones, an on/off switching ratio of 4.0 × 10³, and a raise/decay speed of 97.8/28.9 ms under UVC light. Even without encapsulation, the photodetector keeps a superior stability over ten months. The intrinsically physical insights of the device behaviors are investigated via energy band diagrams, and the charge carrier transfer characteristics of the β-Ga₂O₃/CuI interface are predicted by first principle calculation.

Recent Advances and Challenges in Halide Perovskite Crystals in Optoelectronic Devices from Solar Cells to Other Applications

Organic-inorganic hybrid perovskite materials have attracted tremendous attention as a key material in various optoelectronic devices. Distinctive optoelectronic properties, such as a tunable energy band position, long carrier diffusion lengths, and high charge carrier mobility, have allowed rapid progress in various perovskite-based optoelectronic devices (solar cells, photodetectors, light emitting diodes (LEDs), and lasers). Interestingly, the developments of each field are based on different characteristics of perovskite materials which are suitable for their own applications. In this review, we provide the fundamental properties of perovskite materials and categorize the usages in various optoelectronic applications. In addition, the prerequisite factors for those applications are suggested to understand the recent progress of perovskite-based optoelectronic devices and the challenges that need to be solved for commercialization.

Intrinsic Detectivity Limits of Organic Near-Infrared Photodetectors

Organic photodetectors (OPDs) with a performance comparable to that of conventional inorganic ones have recently been demonstrated for the visible regime. However, near-infrared photodetection has proven to be challenging and, to date, the true potential of organic semiconductors in this spectral range (800-2500 nm) remains largely unexplored. In this work, it is shown that the main factor limiting the specific detectivity (D*) is non-radiative recombination, which is also known to be the main contributor to open-circuit voltage losses. The relation between open-circuit voltage, dark current, and noise current is demonstrated using four bulk-heterojunction devices based on narrow-gap donor polymers. Their maximum achievable D* is calculated alongside a large set of devices to demonstrate an intrinsic upper limit of D* as a function of the optical gap. It is concluded that OPDs have the potential to be a useful technology up to 2000 nm, given that high external quantum efficiencies can be maintained at these low photon energies.

Organic Photodetectors for Next-Generation Wearable Electronics

Next-generation wearable electronics will need to be mechanically flexible and stretchable such that they can be conformally attached onto the human body. Photodetectors that are available in today's market are based on rigid inorganic crystalline materials and they have limited mechanical flexibility. In contrast, photodetectors based on organic polymers and molecules have emerged as promising alternatives due to their inherent mechanical softness, ease of processing, tunable optoelectronic properties, good light sensing performance, and biocompatibility. Here, the recent advances of organic photodetectors in terms of both optoelectronic and mechanical properties are outlined and discussed, and their application in wearable electronics including health monitoring sensors, artificial vision, and self-powering integrated devices are highlighted.

Color-Selective Printed Organic Photodiodes for Filterless Multichannel Visible Light Communication

Future lightweight, flexible, and wearable electronics will employ visible-light-communication schemes to interact within indoor environments. Organic photodiodes are particularly well suited for such technologies as they enable chemically tailored optoelectronic performance and fabrication by printing techniques on thin and flexible substrates. However, previous methods have failed to address versatile functionality regarding wavelength selectivity without increasing fabrication complexity. This work introduces a general solution for printing wavelength-selective bulk-heterojunction photodetectors through engineering of the ink formulation. Nonfullerene acceptors are incorporated in a transparent polymer donor matrix to narrow and tune the response in the visible range without optical filters or light-management techniques. This approach effectively decouples the optical response from the viscoelastic ink properties, simplifying process development. A thorough morphological and spectroscopic investigation finds excellent charge-carrier dynamics enabling state-of-the-art responsivities >10² mA W^-1 and cutoff frequencies >1.5 MHz. Finally, the color selectivity and high performance are demonstrated in a filterless visible-light-communication system capable of demultiplexing intermixed optical signals.

Photomultiplication Type Broad Response Organic Photodetectors with One Absorber Layer and One Multiplication Layer

Broad response organic photodetectors (OPDs) with a photomultiplication (PM) effect are achieved with one absorber layer and one multiplication layer. The response range of the PM-OPDs is primarily determined by materials in the absorber layer, and the external quantum efficiency (EQE) of the PM-OPDs is mainly controlled by the multiplication layer. Here, double-layered PM-OPDs were designed with an ITO/ZnO/PM6:Y6/PC₇₁BM:P3HT (100:5, w/w)/Au structure, where PM6:Y6 is employed as an absorber layer and PC₇₁BM:P3HT is used as a multiplication layer. The optimal PM-OPDs exhibit a broad response covering 350-950 nm. Meanwhile, the optimal PM-OPDs exhibit the largest EQE value of ∼1200% and a maximum specific detectivity (D*) of ∼6.8 × 10^-12 cm Hz^1/2 W^-1 under a 10 V bias. This double-layered approach may be a smart strategy for realizing PM-OPDs with an easily adjustable response range.

Bifunctional Etalon-Electrode To Realize High-Performance Color Filter Free Image Sensor

Organic photodiodes (OPDs), based on organic semiconductors with high absorption coefficients for visible light, are emerging as potential candidates for replacing silicon photodiodes in image sensors, particularly due to the possibility of realizing a thin thickness and exclusion of color filters, both of which can contribute to a dramatically enhanced degree of integration for image sensors. Despite years of research, techniques have not yet been developed that allow the OPD itself to have color selectivity while maintaining a thin (<1 μm) OPD thickness, in combination with a sufficiently high detectivity (>10¹² cm·Hz^0.5/W). To solve this issue, we introduce a concept of "etalon-electrode", which can perform the function of electrode and simultaneously the function of selective wavelength transparency. A strategically designed OPD architecture consisting of an etalon-electrode, a panchromatic organic active layer, and a counter electrode displays well-defined narrowband R-/G-/B-selective detectivity spectra depending on precision-adjusted thickness composition of the etalon-electrode. While a thin thickness of OPD is preserved at less than 800 nm including electrodes, active layer, and other buffer layers for all R-/G-/B-selective OPDs, high average detectivity values over 10¹² cm·Hz^0.5/W are demonstrated. Furthermore, the characteristic of imparting color selectivity by the etalon-electrode enables a more facile full color patterning, such that a prototype of a 10 × 10 image sensor with a pixel pitch of 500 μm is realized, resulting in accurate picturing of a well-defined full color image.

Performance Improvement and Characterization of Spray-Coated Organic Photodetectors

Low dark current organic photodetectors (OPDs) with a conventional structure consisting of poly(3-hexylthiophene) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) as active layer have been fabricated by spray-coating. Tuning the thickness of active layer and thermal annealing process for the spray-coated OPDs results in a remarkable performance with a low dark current density ( J_d) of 2.90 × 10^-8 A/cm² at reverse bias of 1 V. The impact of thermal annealing on the performance of sprayed OPDs is also investigated by the impedance analysis for mechanistic understanding. Our results demonstrate that the optimization of PCBM cluster and interfacial contact between the active layer and the metal electrode tailored by thermal annealing, respectively, could effectively reduce the J_d and increase the sensitivity of sprayed OPDs. The control of PCBM cluster is more important than the interfacial contact between the layers for improving J_d. In addition, structural characterization of the active layer studied by synchrotron small-angle X-ray scattering technique reveals why the spray-coated process can achieve the lowest dark current due to the favorable structure.

Three-Phase Morphology Evolution in Sequentially Solution-Processed Polymer Photodetector: Toward Low Dark Current and High Photodetectivity

Sequentially solution-processed polymer photodetectors (SSP PPDs) based on poly(3-hexylthiophene-2,5-diyl) (P3HT)/[6,6]-phenyl C₇₁-butyric acid methyl ester (PC₇₁BM) are fabricated by depositing the top layers of PC₇₁BM from an appropriate cosolvent of 2-chlorophenol (2-CP)/o-dichlorobenzene (ODCB) onto the predeposited bottom layers of P3HT. By adjusting the ratio of 2-CP/ODCB in the top PC₇₁BM layers, the resulting SSP PPD shows a decreased dark current and an increased photocurrent, leading to a maximum detectivity of 1.23 × 10¹² Jones at a wavelength of 550 nm. This value is 5.3-fold higher than that of the conventional bulk heterojunction PPD. Morphology studies reveal that the PC₇₁BM partially penetrates the predeposited P3HT layer during the spin-coating process, resulting in an optimal three-phase morphology with one well-mixed interdiffusion P3HT/PC₇₁BM phase in the middle of the bulk and two pure phases of P3HT and PC₇₁BM at the two electrode sides. We show that the pure phases form high Schottky barriers (>2.0 eV) at the active layer/electrodes interface and efficiently block unfavorable reverse charge carrier injection by significantly decreasing the dark current. The interdiffussion phase enlarges the donor-acceptor interfacial area leading to a large photocurrent. We also reveal that the improved performance of SSP PPDs is also due to the enhanced optical absorption, improved P3HT crystallinity, increased charge carrier mobilities, and suppressed bimolecular recombination.

Enhanced Thermochemical Stability of CH3NH3PbI3 Perovskite Films on Zinc Oxides via New Precursors and Surface Engineering

Hydroxyl groups on the surface of ZnO films lead to the chemical decomposition of CH₃NH₃PbI₃ perovskite films during thermal annealing, which limits the application of ZnO as a facile electron-transporting layer (ETL) in perovskite solar cells. In this work, we report a new recipe that leads to substantially reduced hydroxyl groups on the surface of the resulting ZnO films by employing polyethylenimine (PEI) to replace generally used ethanolamine in the precursor solutions. Films derived from the PEI-containing precursors are denoted as P-ZnO and those from the ethanolamine-containing precursors as E-ZnO. Besides the fewer hydroxyl groups that alleviate the thermochemical decomposition of CH₃NH₃PbI₃ perovskite films, P-ZnO also provides a template for the fixation of fullerene ([6,6]-phenyl-C61-butyric acid methyl ester, PCBM) owing to its nitrogen-rich surface that can interact with PCBM. The fullerene was used to block the direct contact between P-ZnO and CH₃NH₃PbI₃ films and therefore further enhance the thermochemical stability of perovskite films. As a result, perovskite solar cells based on the P-ZnO/PCBM ETL yield an optimal power conversion efficiency (PCE) of 15.38%. We also adopt P-ZnO as the ETL for organic solar cells that yield a remarkable PCE of 10.5% based on the PBDB-T:ITIC photoactive layer.

Stacking Sequence and Acceptor Dependence of Photocurrent Spectra and Photovoltage in Organic Two-Junction Devices

Both single-junction and tandem organic photovoltaic cells have been well developed. A tandem cell contains two junctions with a charge recombination layer (CRL) inserted between the two junctions. So far, there is no detailed report on how the device will perform that contains two junctions but without a CRL in between. In this work, we report the photocurrent spectra and photovoltage output of the devices that contains two bulk-heterojunctions (BHJ) stacked directly on top of each other without a CRL. The top active layer is prepared by transfer printing. The photocurrent response spectra and photovoltage are found to be sensitive to stacking sequence and the selection of electron acceptors. The open-circuit voltage of the devices (up to 1.09 V) can be higher than the devices containing either junction layer. The new phenomenon in the new device architecture increases the versatility of the optoelectronic devices based on organic semiconductors.